Ron Guth:
The 1855 "Knob on Ear" variety was caused by a small chip that fell out of the die, leaving a void that filled with metal every time a coin was struck. The "Knob", as it is called, is obvious to the naked eye (see above) and is a distinctive and popular variety. The "Knob" is restricted to one 1855 Cent Variety - Newcomb 9.

Mint State 1855 "Knob On Ear" Cents are scarce, but not rare unless they have Red-Brown color. Full Red examples are exceedingly rare.